EP-A 0 181 999 describes a dehydrogenation catalyst which, besides Fe2O3, K2O and MgO, may additionally contain chromium and/or manganese, a compound of cerium, molybdenum or tungsten and CaO. The catalysts mentioned in the examples are calcined at temperatures in the range from 510 to 540xc2x0 C. It is pointed out that the activity of the catalysts, in particular of the selective catalysts, is considerably reduced at relatively high calcination temperatures.
A low calcination temperature (540xc2x0 C.) is also described in EP-A 0 177 832 for magnesium-containing catalysts based on Fe2O3 and K2O.
DE-A 28 15 812 describes dehydrogenation catalysts which consist of mixtures of iron oxide, potassium oxide, vanadium oxide and, if desired, chromium oxide. The selectivity and/or conversion rate to unsaturated hydrocarbons from saturated compounds is said to be improved by small amounts of oxygen-containing compounds of aluminum, cadmium, copper, magnesium, manganese, nickel, uranium, zinc or a rare earth and mixtures thereof.
DE 38 21 431 describes a K2Fe22O34-containing catalyst which is calcined at 900xc2x0 C. For the preparation, exclusively iron-oxide and a potassium compound are employed. The calcination product is subsequently washed and filtered, the resultant product being lamellar plates having a diameter of from 0.5 to 5 xcexcm.
It is an object of the invention to provide a catalyst having improved activity and selectivity in the dehydrogenation of ethylbenzene to styrene. The catalyst should in addition have high mechanical and chemical stability and a good shelf life.
We have found that this object is achieved by a catalyst comprising iron oxide, potassium oxide, a magnesium compound and a cerium compound, where the catalyst has one or more Fe/K phases K2O.Fe2O3 1:n, where n is a natural number from 1 to 11, in particular one of the phases K2O.Fe2O3 1:4 (K2Fe8O13), K2O.Fe2O3 1:5 (K2Fe10O16) and/or K2O.Fe2O3 1:11 (K2Fe22O34).
The special structural properties compared with the known catalysts include large pore diameters at the same time as high mechanical stability, a large internal surface area, a low weight per liter and the significant formation of X-ray detectable Fe/K phases K2O.Fe2O3 1:4 and/or K2O.Fe2O3 1:11. The best magnesium-containing catalysts known to date do not, owing to the low calcination temperature, contain Fe/K phases, with the exception of small amounts of K2Fe2O4, but instead contain only Fe2O3 (hematite) as iron constituent. The Fe/K phases K2O.Fe2O3 1:4 and K2O.Fe2O3 1:11 apparently only form from 750xc2x0 C.
The Fe/K phases can be determined radiographically. The lattice plane separations and relative intensities of the Fe/K phases K2O.Fe2O3 1:5 (K2Fe10O16) and/or K2O.Fe2O3 1:11 (K2Fe22O34 in Table 3. Owing to the inclusion of magnesium, cerium and possibly further promoter and added metals in the Fe/K phases, the reflections may be slightly shifted compared with the pure Fe/K phases.
Preferred catalysts according to the invention have reflections for lattice plane separations in the following ranges:
Particularly preferred catalysts have a 1st reflection in the range from 11.70 xc3x85 to 11.90 xc3x85, in particular from 11.74 xc3x85 to 11.87 xc3x85, and second reflection at from 5.85 xc3x85 to 5.95 xc3x85, in particular from 5.89 xc3x85 to 5.93 xc3x85.
Preferred catalysts comprise 50-90% by weight of iron, calculated as Fe2O3, from 1 to 40% by weight of potassium, calculated as K2O, from 5 to 20% by weight of cerium, calculated as Ce2O3, and from 0.1 to 10% by weight of magnesium, calculated as MgO.
In addition to magnesium and cerium, the catalyst may furthermore comprise one or more further conventional promoters for increasing the selectivity, activity or stability in conventional concentrations. Suitable promoters are compounds of elements selected from the group consisting of Be, Ca, Sr, Ba, Sc, Ti, Zr, Hf, V, Ta, Mo, W, Mn, Tc, Re, Ru, Os, Co, Rh, Ir, Ni, Pd, Pt, Cu, Ag, Au, Zn, Cd, Hg, Al, Ga, In, Tl, Ge, Sn, Pb, Bi, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu, which can be used individually or in mixtures. Preferred additional promoters are compounds selected from the group consisting of Ca, V, Cr, Mo, W, Ti, Mn, Co and Al. Particularly preferred additional promoters are Ca, V, Cr, Mo and W. The additional promoters are preferably added in amounts of in each case from 0 to 15% by weight, in particular from 1 to 10% by weight, calculated as the most stable oxides.
Potassium can be replaced in part or full by equivalent amounts of other alkali metals, for example cesium or sodium.
The novel catalyst preferably comprises iron, potassium, cerium and magnesium and, as further elements, tungsten, molybdenum and calcium. Favorable catalysts are furthermore those which contain vanadium.
The addition of vanadium further increases the selectivity of the catalysts. The addition of vanadium (0.1-10% by weight) is therefore very advantageous.
Elements such as Cr, Al, Ti, Co, Li and Zn are generally present in the catalysts according to the invention in secondary amounts, for example from 0 to 2% by weight, in particular from 0 to 1% by weight, in each case as the oxide.
However, the catalysts preferably contain no chromium.
For example, a novel catalyst comprises, in the ready-to-use state:
50-90% by weight, in particular 60-80% by weight, of iron, calculated as Fe2O3;
1-40% by weight, in particular 5-15% by weight of potassium, calculated as K2O;
5-20% by weight, in particular 6-15% by weight, of cerium, calculated as Ce2O3;
0.1-10% by weight, in particular 1-5% by weight, of magnesium, calculated as MgO;
0-10% by weight, in particular 0.1-4% by weight, of calcium, calculated as CaO;
0-10% by weight, in particular 0-5% by weight, of tungsten, calculated as WO3;
0-10% by weight, in particular 0-5% by weight, of molybdenum, calculated as MoO3;
0-10% by weight, in particular 0.1-4% by weight, of vanadium, calculated as V2O5,
with the proviso that at least 0.1% by weight, in particular 1% by weight, of tungsten or molybdenum is present.
The potassium compound used is preferably potassium carbonate, potassium hydroxide or another potassium compound which can be decomposed at elevated temperatures, such as potassium oxalate. It is also possible to use a potassium compound which contains the proposed promoter (i.e. as the corresponding anion or as a double salt).
The vanadium compound used is preferably V2O5, ammonium vanadate or alkali metal vanadates.
The magnesium compound used is preferably Mg(OH)2, MgO, MgCO3 or magnesium bicarbonate.
The cerium compound used is preferably Ce2O3, cerium oxalate or cerium carbonate.
The molybdenum compound used is preferably MoO3, H2MoO4 or ammonium molybdate.
The tungsten compound used is preferably WO3, H2WO4 or ammonium tungstite.
The aluminum compound used is preferably Al OOH or Al2O3.
The calcium compound used is-preferably CaO, CaCO3 or Ca(OH)2.
The novel catalyst is prepared predominantly from xcex1-Fe2O3 instead of the FeOOH preferred in EP-A-0195252 and contains an amount of cerium of up to 20%, calculated as Ce2O3, which is increased over the recommendation given therein of 3-6% by weight of Ce2O3. The increased amount of cerium used in the novel catalysts results in an improvement in the activity and long-term stability.
Preference is given to catalysts which have been obtained using xcex1-Fe2O3 (hematite) having a particle size of greater than 0.3 xcexcm and pore diameters of corresponding size. The catalysts preferably have a mean pore diameter of greater than 0.35 xcexcm, in particular greater than 0.40 xcexcm. Pure iron oxide hydroxide (FeOOH) is less suitable for the preparation of the catalysts. Studies on corresponding comparative catalysts show that these have neither the desired large pore diameter nor satisfactory mechanical stability.
Instead of pure xcex1-Fe2O3 (hematitie), however, it is possible to employ mixtures of xcex1-Fe2O3 and xcex1-FeOOH (goethite) as the iron component, so long as at least 50% by weight of xcex1-Fe2O3 are used in the preparation. Preference is given to mixtures of from 60 to 90% by weight of xcex1-Fe2O3 and from 10 to 40% by weight of xcex1-FeOOH. Although the catalyst prepared by the process according to the invention from a mixture of, for example, 70% by weight of Fe2O3 and 30% by weight of FeOOH has on average somewhat smaller pore radii and somewhat lower mechanical stability, it has, on the other hand, a significantly larger internal surface area than the catalyst prepared only from Fe2O3. The consequence is a further improvement in the activity.
In principle, the preparation follows the process given in EP A 0 195 252, but the calcination is carried out at significantly higher temperatures. The improvement produced by the novel catalysts over these known catalysts and those described elsewhere is apparently due to the calcination at temperatures above 750xc2x0 C., preferably at from 760 to 1000xc2x0 C., in particular at from 800 to 900xc2x0 C., and the use of a certain iron modification, magnesium and an increased amount of cerium in an otherwise comparable process. The higher calcination temperature together with the higher amount of cerium and the content of magnesium results in novel, unusual properties of the catalysts and at the same time in a significant improvement in the activity and selectivity. It has been found that this catalyst has excellent productivity and long-term stability as well as good mechanical stability at low vapor/EB ratios (up to V/EB=1.0 kg/kg). Owing to the lower energy consumption of the process operated therewith, it therefore also offers economic advantages over the known catalysts. However, even at the higher vapor/EB ratios of 1.1-1.5 kg/kg which are currently usual, the new catalyst achieves better productivity.
The invention also relates to a process for the dehydrogenation of alkylaromatic or aliphatic hydrocarbons to the corresponding alkenes using the novel catalysts.
Particular preference is given to the dehydrogenation of ethylbenzene to styrene. However, the catalysts may advantageously also be employed for the dehydrogenation of 1,1-diphenylethane (DPEA) to 1,1-diphenylethene (DPE). DPE is in demand as a raw material for styrene copolymers of increased heat resistance.
The following details apply to the preparation of the novel catalysts:
Mixing of the Starting Materials and Preparation of a Shapeable Material
Intimate mixing of the starting materials is important. This can be achieved simply by dry mixing or by suspending the starting materials in water and spray-drying the resultant suspension. During all mixing operations, it is advantageous for all constituents, with the exception of the iron oxide, to be in very finely divided form. After addition of water, a shapeable material is obtained from the mixtures by compounding. The shaping to tablets can, by contrast, be carried out using a dry mix of the constituents.
Production of Moldings
Moldings can be produced by extrusion or by tableting the dry spray powder or a mixture in a tablet press. In this case, it may be beneficial to add tableting auxiliaries (for example graphite or various stearates) to the tableting material. Suitable moldings are extrudates having various geometries, preferably solid extrudates having a diameter of, for example, 3-6 mm. Also suitable are, for example, hollow extrudates or rib or star extrudates (i.e. extrudates having external longitudinal ribs or a star-shaped cross section) or ring tablets having a central hole. The shapeability can be modified using auxiliaries such as stearates, Walocel, starch or the like.
Drying
The drying can be carried out continuously or batchwise. Continuous drying can be carried out using belt dryers and batch drying using tray ovens. On an industrial scale, suitable equipment is that which is suitable for industrial-scale drying processes, such as belt dryers or tray ovens. The usual drying temperatures are 80-140xc2x0 C. Higher drying temperatures may result in undesired cracking of the moldings owing to an excessive drying rate. Lower drying temperatures are possible, but extend the drying time.
Conditioning; Calcination
After the drying, the moldings are firstly heated at 250-350xc2x0 C. for about 2 hours (xe2x80x9cconditioningxe2x80x9d), then at 750-1000xc2x0 C. for 1-2 hours (xe2x80x9ccalcinationxe2x80x9d). In the case of production on an industrial scale, the conditioning and calcination can be carried out in a single operation, for example in a rotating tube with various heating zones. The temperature is then increased in steps from, for example, 250xc2x0 C. to 800-900xc2x0 C. After exiting from the rotating tube, the moldings are allowed to cool. Fragments and fine dust are separated off by screening and discarded.